BLOOD PRESSURE. 73 



the radial artery the pressure in the rubber bag is raised above 

 that necessary to produce obliteration of the pulse. Now no 

 sound can be heard over the brachial artery at the elbow. By 

 gently releasing the valve the pressure is gradually lowered. 

 Just as soon as the pressure becomes less than systolic pressure, 

 the blood is forced through the occluded arteries and a distinct 

 thud is heard at each heart beat. At the same time or often 

 after the pressure has been lowered 5-10 mm. more, the pulse 

 is felt at the wrist. The first sound is never below the tactile 

 indication; if it is, some error has been made in the application 

 of the apparatus, usually the stethoscope. The pressure read 

 at the time that the first sound is heard is systolic pressure. 



As the armlet pressure is further lowered, the sound be- 

 comes progressively louder and sometimes resembles a murmur 

 in character, and then, rather suddenly, it becomes feebler and 

 at the same time duller. At this point, the manometer indicates 

 the diastolic pressure. After this sudden sound, the sound may 

 continue to be heard for a longer or shorter time as the armlet 

 pressure continues to fall. 



The difference between systolic and diastolic pressure is the 

 pulse pressure. 



Determine the heart rate, systolic and diastolic pressures 

 under the following conditions in as many different subjects as 

 possible: 



Supine position, sitting, standing and immediately after 

 vigorous exercise as running up and down stairs. How does the 

 pulse pressure change under these conditions? 



PRINCIPLES OF HAEMODYNAMICS. 



The blood propelled by the heart, circulates in a closed system 

 of tubes. Both the output of the heart and the response of the 

 vessels affect the rate of flow. Increase the output of the heart 

 either by a greater number of contractions per unit of time or by 

 increasing the amplitude; either change will produce a greater flow 

 of blood. 



If the blood circulated in perfectly rigid vessels the pressure in 

 the system would rise to a maximum during the contraction of the 



